The present invention relates to golf balls having a core and a cover layer, and more particularly to golf balls endowed with exceptional scuff resistance, a large rebound and a suitable spin rate on approach shots.
Ionomer resins and the trans-polyisoprene rubber known by the name of balata rubber have been used to date as the cover material in golf balls. Golf balls made with balata rubber in particular are preferred by golfers who value controllability and the feel of the ball when hit with a club.
However, because a balata cover is inferior to an ionomer cover in terms of resilience, distance traveled by the ball and scuff resistance on shots taken with an iron, there is much room for improvement. A need thus exists for new golf balls endowed not only with a large rebound and distance, but also improved scuff resistance, controllability and feel upon impact.
JP-A 6-319830 discloses a two-piece solid golf ball in which the cover layer is made of a relatively soft ionomer resin. Yet, there is still room for improvement in the scuff resistance of the ball.
JP-A 9-215778 describes a two-piece solid golf ball having a urethane cover. Here too, the scuff resistance leaves something to be desired.
JP-A 2001-70478 discloses technology for obtaining golf balls in which the cover layer is formed of a rubber composition containing a polybutadiene having a cis-1,4 structure as the base rubber, and containing also specific amounts of xcex1,xcex2-unsaturated carboxylic acid, metal oxide and polymerization initiator. Such a cover layer provides the ball with an excellent controllability and a good feel comparable to that of a ball having a cover made of balata rubber, and also gives it a large rebound and good flight performance and scuff resistance. However, such golf balls are sometimes overly receptive to spin, and thus leave something to be desired with regard to their controllability in a head wind, particularly when used by a professional golfer or a skilled amateur.
It is therefore an object of the invention to provide a golf ball which has the degree of spin desired by professional golfers, skilled amateurs, and other players who aspire to a high level of proficiency, and which also has a good rebound and excellent scuff resistance.
We have discovered that, in solid golf balls having a cover layer made of a rubber composition containing polybutadiene with at least a 40 wt % cis-1,4 structure, methacrylic acid, metal oxide and a polymerization initiator, by additionally compounding a polybutadiene of a specific structure in the rubber composition, the hardness of the cover layer can be improved while retaining such qualities as the good scuff resistance inherent to the crosslinked form of the rubber composition. We have thus found that golf balls with a cover layer made of a rubber composition which includes such a polybutadiene of a specific structure have the degree of spin desired by professional golfers, skilled amateurs, and other players who aspire to a high level of proficiency, and also have an excellent scuff resistance and a good rebound.
Accordingly, the invention provides the following golf balls.
(1) A solid golf ball having a core and a cover layer, characterized in that the cover layer is formed by carrying out a crosslinking reaction on a rubber composition comprising
100 parts by weight of a base rubber which is composed mainly of a mixture of (A) a polybutadiene of at least 40% cis-1,4 structure with (B) a syndiotactic 1,2-polybutadiene,
(C) at least 5 parts by weight of methacrylic acid,
(D) at least 5 parts by weight of a metal oxide, and
(E) at least 0.1 part by weight of a polymerization initiator.
(2) The solid golf ball of (1) above, characterized in that 100 parts by weight of the base rubber is composed of 50 to 95 parts by weight of component A, 5 to 40 parts by weight of component B, and (F) 0 to 20 parts by weight of a polyisoprene rubber of at least 40 wt % cis-1,4 structure; and
the rubber composition includes 5 to 40 parts by weight of component C, 5 to 40 parts by weight of component D, and 0.1 to 5 parts by weight of component E per 100 parts by weight of the base rubber.
(3) The solid golf ball of (1) or (2) above, characterized in that the ball surface and the core have a difference in Shore D hardness therebetween such that xe2x88x923xe2x89xa6[(ball surface hardness)xe2x88x92(average core hardness)].
(4) The solid golf ball of any one of (1) to (3) above which is a two-piece solid golf ball.
(5) The solid golf ball of any one of (1) to (4) above, characterized in that the cover layer has an overall thickness of at most 2.0 mm.
(6) The solid golf ball of any one of (1) to (5) above, characterized in that the ball has a USGA standard test-based initial velocity of 76.5 to 77.7 m/s.